The production of hollow fibers by melt spinning or wet spinning has long been known. The processes mentioned in numerous patents are based essentially on three approaches.
In the first method, a molten polymer, for example a polyester, is spun from nozzles as adjacent arcuate segments. Synthetic hollow fibers are produced by swelling the molten polymer beneath the nozzle and allowing the edges of the arcuate segments to coalesce into a continuous form. In the second method, a hollow needle positioned in the center of the orifice is used, gaseous substances or fillers being pumped through the hollow needle. The polymer flows round the needle and the gas fills the central void and maintains the form until the polymer has cooled. Hollow viscose filaments, in particular, are produced in this way and castor oil, for example, may be used as lumen-filling medium. Lastly, in the third method, a solid pin is positioned in the nozzle orifice. This is generally a difficult spinning process as the polymer wishes to assume a closed form. The process is particularly suitable for cross-section modifications, but air has to be supplied to the end of the pin or a vacuum has to be applied to produce hollow fibers.
Hollow filaments and fibers have found many applications. Thus, for example, they are used for the desalination of sea water, for the purification of liquids and gases, in ion exchanger, for reverse osmosis, dialysis and ultrafiltration (artificial kidneys) and, because of the low weight and the high bulk thereof, for comfortable clothes. In particular, the purification of substances, for example industrial gases, has recently come to the fore. Comprehensive articles about the production and importance of synthetic hollow fibers may be found in the Encyclopedia of Polymer Science and Technology, 15, (1971), Pages 258-272, in Acta Polymerica, 30, (1979), Pages 343-347 and in Chemical Engineering, February 1980, Pages 54-55.
There have also been numerous attempts to produce hollow acrylic fibers from a spinning solution by a dry spinning process. However, owing to the problems encountered, no commericial process for the production of hollow acrylic fibers by this technique has previously been disclosed.
For the present purposes, the term "hollow fibers" refers to fibers having an internal, linear, continuous longitudinal channel.
Although acrylonitrile polymers may be converted to hollow fibers relatively simply by the wet spinning technique by one of the above-mentioned methods, this leads to considerable difficulties in a dry spinning process owing to a different filament formation mechanism. In a wet spinning process, filament formation is effected by coagulation of the spinning solution in an aqueous precipitating bath containing a solvent for polyacrylonitrile, the precipitating bath concentration, temperature and additional coagulating agent, such as aqueous salt solutions, may be varied within wide limits. Thus, for example, German Offenlegungsschrift No. 2,346,011 describes the production of hollow acrylic fibers by the second wet spinning method using aqueous DMF as precipitating bath and German Offenlegungsschrift No. 2,321,460 uses aqueous nitric acid, the filaments being spun from nozzles having annular orifices and a liquid being introduced into the center of the annular orifice as an internal precipitant.
In attempting to apply the three methods to a dry spinning process, considerable difficulties are encountered as, when spinning from a spinning solution, only a proportion of the solvent has to evaporate after issuing from the nozzle in order for a thread to be formed and solidify. Owing to the high production costs and the difficult process control when producing hollow acrylic fibers by dry spinning from spinning solutions, the second and third methods were not pursued.
When attempting to produce hollow fibers according to the first method using profile nozzles having adjacent segmental arcuate orifices by the dry spinning process, only dumbbell-shaped or irregular random cross-sections are generally obtained which have uneven air inclusions. If the concentration of polymer solids is increased in order to obtain the predetermined cavity profile by increasing the structural viscosity, unexpected problems arise. The increase in the solids content is subject to limits owing to the gelation, flowability and management of the spinning solutions. Thus, for example, an acrylonitrile copolymer having a chemical composition of 93.6% of acrylonitrile, 5.7% of acrylic acid methyl ester and 0.7% of sodium methallyl sulphonate and a K-value of 81 may only be dissolved and spun into threads in a spinning solvent, such as dimethylformamide, to a maximum solids content of 32%, by weight. If an attempt is made to raise further the solids content, and spinning solutions gel during cooling at temperatures of from 50.degree. to 80.degree. C., rendering disturbance-free spinning impossible.